Hydraulic or hydrostatic transmissions, for example, for on-highway vehicles such as trucks, have received greater interest in recent years as the price of fuel has increased sharply. Hydrostatic transmissions (HSTs) have an advantage over conventional hydrodynamic and gear transmissions in regard to fuel efficiency due to their ability to recover kinetic energy during braking via hydraulic accumulators, and also due to the fact that the prime mover or engine is uncoupled from the drive wheels, thus allowing the engine to always operate at its most efficient operating range, regardless of vehicle speed and torque demands.
In order for HSTs to successfully compete with conventional transmissions, their increased fuel efficiency must not be offset by other factors, such as decreased reliability, lower performance, increased cost, weight and complexity, etc. One of the shortcomings of HSTs is their relatively lower transmission efficiency. Although vehicles having HSTs with brake energy recovery can exhibit reduced fuel usage of up to fifty percent, depending on driving conditions, the transmission itself is less efficient. Therefore, supplementary heat exchangers for cooling the transmission fluid are generally employed. These devices increase cost, weight, and number of fluid connections and thus leak points, and make the system more difficult to package on the vehicle.
HSTs with brake energy recovery employ hydraulic accumulator(s) to store energy from braking. During a braking event, the vehicle's drive wheels provide the energy for the HST to pump high pressure fluid into the accumulator, which acts to slow the vehicle. In order to maximize the amount or energy that can be recovered, relatively large accumulators must be used, with the size thereof being largely dependent upon the size of the vehicle. This pressurized fluid is later discharged through the transmission to drive the wheels, with the fluid, used to fill the accumulator, being supplied by a low pressure reservoir. When the accumulator is subsequently discharged to propel the vehicle, the fluid is transferred back to the low pressure reservoir. Therefore, the volume of the low pressure reservoir must approximately equal the volume of the high pressure accumulator.
A major reason for the relative inefficiency of HSTs is that the fluid is always passed through two hydraulic pump/motors. For instance, during a braking event, the HST pumps high pressure fluid into the accumulator, which adds heat to the fluid when the accumulator is discharged through the HST. The same thing occurs when power is supplied by the engine, since the engine-driven pump adds its inefficiency to the fluid in terms of heat, with the pump/motor further adding its own inefficiency. Although each pump/motor, by itself, typically has good efficiency (85-95%), the overall efficiency can be as low as 85%×85%=72% during worst case conditions. Even if the average efficiency throughout all driving conditions is 90% per pump/motor, the overall transmission efficiency is only 81%. For a vehicle that operates at high power levels for a large portion of its duty cycle, such as a refuse truck, this heat load can be on the order of 50 horsepower. Without means to cool the transmission fluid, the system would overheat very quickly. A typical means used to cool the transmission fluid is illustrated in prior art FIG. 1. Further details, in terms of such transmission technology, are set forth in a paper entitled “Cumulo Hydrostatic Drive—a Vehicle Drive with Secondary Control”, which was presented by its author, Conny Hugosson, at The Third Scandinavian International Conference on Fluid Power, on May 25-26, 1993, in Linköping, Sweden.
The patent literature includes a large number of patents pertaining to heat transfer apparatuses that include but are not limited to: U.S. Pat. No. 3,688,940 to Knight et al.; U.S. Pat. No. 4,368,775 to Ward; U.S. Pat. No. 5,144,801 to Scanderbeg et al.; U.S. Pat. No. 5,513,490 to Howell et al.; U.S. Pat. No. 5,718,281 to Bartalone et al.; U.S. Pat. No. 6,261,448 B1 to Merchant et al.; and U.S. Pat. No. 6,736,605 B2 to Ohashi et al. However, none of these prior art structures set forth or suggest a hydraulic reservoir consisting of spaced inner and outer shells with a suitable intermediate gap wherein the outer shell is provided with external fins to effect efficient heat transfer, with the working fluid being drawn fro the reservoir and the being returned to the reservoir through the integrated heat exchanger.